Wikipedia on Roller Chains:
The first digit(s) indicate the pitch of the chain in eighths of an inch, with the last digit being 0 for standard chain, 1 for lightweight chain, and 5 for bushed chain with no rollers. Thus, a bicycle chain with half inch pitch would be a #40 while a #160 sprocket would have teeth spaced 2 inches apart, etc.
This
site explains some about sizes. It doesn't have 25 on there, but it discusses the difference between #35 and 40's chain.
As far as the clutch goes, I'm not sure what to do yet. I'm leaning toward getting the bigger clutch off ebay just cause I don't want to have to worry about buying a new chain and sprocket if I can help it.
Thanks for all the input guys.